The invention relates to an electromagnetic actuator, in particular for a trip device of an electrical switchgear apparatus.
FIG. 7 represents a known actuator of the state of the technique. This actuator 110 comprises a fixed magnetic circuit 112, made of ferromagnetic material, formed by a shell closed at one of its ends on a fixed core 122. A movable assembly 114 is designed to slide parallel to a fixed geometrical axis and comprises a mobile core 116 and a rod 118 associated to the mobile core and passing axially through an opening of the fixed core 122. A spiral-wound compression spring 140 biases the movable assembly 114 to a rest position.
A coiled winding with two fixed coils 130, 132 is fitted inside the shell and surrounds the mobile core 16. This coiled winding is designed to generate a magnetic control flux in the magnetic circuit so as to move the movable assembly towards the fixed core against the action of the spring 140 to an active position.
Such a device is conventionally used in shunt releases (MX) and as closing electromagnet (XF) of a circuit breaker. In case of actuation of the electromagnet, an inrush current flowing in the two coils 130, 132 causes movement of the mobile core 116, and consequently of the rod 118, which then protrudes outwards thus enabling either opening of the associated circuit breaker in the case of a shunt release (MX) or closing of the circuit breaker in the case of a closing electromagnet (XF). It is therefore the electromagnetic energy supplied by the coils 130, 132 during the inrush phase which causes actuation of the circuit breaker. In other words, the rod 118 must be able to perform the mechanical work necessary for movement of the latch to which it is associated, this work corresponding to the energy supplied by the coils 130, 132 in the inrush phase. The inrush phase is followed by a holding phase during which only one of the two coils 130, 132 is supplied. A minimum axial air-gap is maintained by fitting a spacer 141 between the mobile core and the fixed core. When the voltage is lower than a dropout threshold, the current flow in the coil winding is interrupted and the mobile core 116 is separated from the fixed core by the action of the spring 140. As switching to this position does not have any action on the circuit breaker, the power of the spring is relatively indifferent in this phase. The spacer 141 prevents the mobile core 116 from remaining "stuck" to the fixed core 122 due to the remanence effect of the magnetic circuit when the power supply to the coil is interrupted.
In a device of this kind, the dimensioning of the different elements, in particular of the spring and the minimum air-gap in the active position, is difficult. The potential energy of the contracted spring, which has to return the movable assembly to the rest position on its own, must be great enough to overcome the remanent magnetic energy. The presence of the air-gap enables the sticking effect to be limited but induces a risk of nuisance unsticking, i.e. of an involuntary return to the rest position, in particular in response to a mechanical shock on the rod or a large vibration of the movable assembly. If it is chosen to reduce the air-gap, the potential energy of the return spring then has to be increased accordingly, so that the inrush energy necessary to move the movable assembly to the active position is also increased.